Current gravitational wave (GW) detectors are Michelson-type kilometre-scale laser interferometers measuring the distance changes between in vacuum suspended mirrors. The sensitivity of these detectors is limited by quantum noise of the laser light. Since 2010, GEO600 has been using so-called nonclassical light that has a ‘squeezed’ quantum noise, and it has been demonstrated that squeezed light is able to increase the GW detector sensitivity at shot-noise limited frequencies. In future, squeezed light might be used to further improve the sensitivity of GW detectors, then also at radiation pressure noise dominated frequencies and at frequencies where photon-phonon scattering is limiting the detectors performance.